The present invention relates generally to a hydraulic brake system for an automative vehicle, and more particularly to a vehicle anti-skid brake system for efficient braking of the vehicle without skidding of its wheels, wherein skidding conditions of the wheels are sensed, and a hydraulic braking pressure applied to brake cylinders for the wheels is controlled in response to the sensed skidding conditions of the wheels.
In a common hydraulic brake system for an automotive vehicle, hydraulic braking pressure generated by a master cylinder upon activation of a brake operating member of the vehicle is applied through brake fluid passages to brake cylinders disposed adjacent to respective wheels, so that the brake cylinders are operated with the hydraulic pressure to apply a brake to the wheels and thereby restrain their rotation. In recent years, it has been the practice to provide such a brake system with a solenoid-operated directional control valve, which is disposed in the fluid passage to effect communication of the brake cylinders selectively with the master cylinder or a brake fluid reservoir of the system. This selective communication is achieved by operating the directional control valve under automatic control by a suitable control device which is adapted to sense a skidding or locked condition of each wheel under braking and regulate the operation of the directional control valve according to the sensed skidding condition of the wheel.
The above indicated anti-skid brake system, wherein the actually sensed skidding conditions of the wheels are used to control the hydraulic pressure to be applied to the respective brake cylinders for the wheels, is capable of efficient braking of the vehicle without skidding of the wheels during braking of the wheels. However, the activation of the directional control valve may cause a subsequent abrupt or sudden supply of a pressurized fluid to the brake cylinders, i.e., the control valve may fail to accomplish a smooth variation in operating pressure of the brake cylinders. For this reason, a suitable restrictor is generally provided in the fluid passage.
On the other hand, the provision of a restrictor for such purpose is disadvantageous in several aspects. Described more specifically, upon initiation of a brake application, a relatively large amount of brake fluid should be supplied to the brake cylinder before a piston in the brake cylinder starts the application of an effective braking force to the wheel, that is, a certain amount of brake fluid is needed before brake clearance has been eliminated, and before a piston cup of the brake cylinder and other components of the system which are subject to deformation under a relatively low pressure, have undergone deformation to the extent that permits the brake cylinder to initiate actual brake application to the wheel. In such circumstances, the use of a restrictor in the fluid passage is not desired. An excessive restriction of pressure or flow of the fluid by the restrictor contributes to prevention of a smooth, rapid supply of the brake fluid to the brake cylinder, which leads to a delayed braking action of the brake cylinder. Further, with the brake fluid restricted in an excessive degree, a relatively high pressure is developed in a pressure chamber of the master cylinder before a piston cup of the master cylinder piston has passed a relief port (compensating port) which communicates with the pressure chamber and a brake fluid reservoir. The pressure buildup in the pressure chamber at this time may cause the piston cup to be moved within the pressure chamber with a portion of the cup being forced into the opening of the relief port, whereby the piston cup of the master cylinder is likely to be subject to local wear or damage.
Stated another way, a restriction of fluid flow through the fluid passage leading to the master cylinder is not desired during an initial stage of brake application wherein a large amount of brake fluid is required, but such a restriction is desired when the anti-skid device is operational and the solenoid-operated directional control valve is being switched.
Traditionally, a degree of restriction of the brake fluid by a restirctor is selected so as to obtain a compromise between the above two different requirements. However, these two requirements are inherently incompatible, and difficult to satisfy at the same time. Consequently, one of the requirements is more or less sacrificed for the sake of the other.